Glass cup + Water not rendering correctly, weird lines
Lines in transparent materials are a telltale sign of intersections with other objects. This was no exception.
To verify that there were intersections between the liquid and cup objects, I took these steps to generate a mesh that resembles the intersections:

Tab out of edit mode. This only works in object mode.
Select the liquid.
Hit the &quot;Modifier Properties&quot; tab of the properties window.
Create a new Boolean modifier (it's under the Generate group) with the following properties.


Operation: Difference
Object: Cup
Overlap Threshold: 0

Below is the screenshot of the results:

Yep, the cup and liquid definitely intersect. And notice how those lines match the lines in the render.
One way to fix the problem is by shrinking the liquid in the x and y axes. Here's how I did it:

Remove the Boolean modifier (that was just to see the intersections)
Select the liquid and only the liquid.
Tab into edit mode
Hit a to select all vertices
Hit s to scale them
Hit shift and z to lock the transformation in the Z axis.
Enter 0.99.
Tab out of edit mode.

Now the cup should look like this:

At first glance, it doesn't look like anything changed. But when you render the scene...

...no more lines!
One other thing to note is that the cup has some duplicate vertices. Make sure to use the &quot;Merge by Distance&quot; tool occasionally to clean those up.